Ink jet printing has the inherent advantage of being a plain paper compatible, direct marking technology. However, the technology has been slow to mature, at least in part because most "continuous stream" and "drop on demand" ink jet print heads include nozzles. Although steps have been taken to reduce the manufacturing cost and increase the reliability of these nozzles, experience suggests that the nozzles will continue to be a significant obstacle to realizing the full potential of the technology.
Others have proposed nozzleless liquid ink print heads, including ultrasonic print heads, to avoid the cost and reliability disadvantages of conventional ink jet printing while retaining its direct marking capabilities. See, for example, Lovelady et al. U.S. Pat. No. 4,308,547, which issued Dec. 24, 1981 on a "Liquid Drop Emitter." Furthermore, significant progress has been made in the development of relatively low cost, nozzless, ultrasonic print heads. See a copending and commonly assigned United States patent application of C. F. Quate et al, which was filed Sept. 16, 1985 under Ser. No. 776,291 on a "Leaky Rayleigh Wave Nozzeless Droplet Ejector".
Capillary surface waves (viz., those waves which travel on the surface of a liquid in a regime where the surface tension of the liquid is such a dominating factor that gravitational forces have negligible effect on the wave behavior) are attractive for liquid ink printing and similar applications because of their periodicity and their relatively short wavelengths. However, it appears that they have not been considered for such applications in the past. As a practical guideline, surface waves having wavelengths of less than about 1 cm. are essentially unaffected by gravitational forces because the forces that arise from surface tension dominate the gravitational forces. Thus, the spatial frequency range in which capillary waves exist spans and extends well beyond the range of resolutions within which non-impact printers normally operate.
As is known, a capillary wave is generated by mechanically, electrically, acoustically, thermally, pneumatically, or otherwise periodically pertubing the free surface of a volume of liquid at a suitably high frequency, .omega..sub.e. In the presence of such a perturbation, a traveling capillary surface wave having a frequency, .omega..sub.tc, equal to the frequency, .omega..sub.e, of the perturbance (i.e., the excitation frequency) propagates away from the site of the perturbance with a wave front geometry determined by the geometry of the perturbing source. In another variation, capillary waves can be generated with a parametric process. When the amplitude of the surface perturbation equals or exceeds a so-called onset amplitude level, one or more capillary waves are generated on the free surface of the liquid. Standing waves are produced by a parametric excitation of the liquid, with a frequency, .omega..sub.sc, equal to one half the excitation frequency (i.e., .omega..sub.sc =.omega..sub.e /2). This parametric process is described in substantial detail in the published literature with reference to a variety of liquids and a wide range of operating conditions. See, for example, Eisenmenger, W., "Dynamic Properties of the Surface Tension of Water and Aeguous Solutions of Surface Active Agents with Standing Capillary Waves in the Frequency Range from 10 kc/s to 1.5 Mc/s", Acustica, Vol. 9, 1959, pp. 327-340.
While the detailed physics of traveling and standing capillary surface waves are beyond the scope of this invention, it is noted that waves of both types are periodic and generally sinusoidal at lower amplitudes, and that they retain their periodicity but become non-sinusoidal as their amplitude is increased. As discussed in more detail hereinbelow, printing is facilitated by operating in the upper region of the amplitude range, where the waves have relatively high, narrow crests alternating with relatively shallow, broad troughs.
Standing capillary surface waves have been employed in the past to more or less randomly eject droplets from liquid filled reservoirs. For example, medicinal inhalants are sometimes dispensed by nebulizers which generate standing waves of sufficient amplitude to produce a very fine mist, known as an "ultrasonic fog". See Boucher, R. M. G. and Krueter, J., "The Fundamentals of the Ultrasonic Atomization of Medicated Solutions," Annals of Allergy, Vol. 26, Nov. 1968, pp. 591-600. However, standing waves do not necessarily produce an ultrasonic fog. Indeed, Eisenmenger, supra at p. 335, indicates that the excitation amplitude required for the onset of an ultrasonic fog is about four times the excitation amplitude required for the onset of a standing capillary wave, so there is an ample tolerance for generating a standing capillary surface wave without creating an ultrasonic fog.
As will be appreciated, there are fundamental control problems which still have to be solved to provide a traveling or standing capillary surface wave printer. In contrast to the non-selective ejection behavior of known capillary wave droplet ejectors, such as the aforementioned nebulizers, the printing of a two dimensional image on a recording medium requires substantial control over the spatial relationship of the individual droplets which are deposited on the recording medium to form the image, For instance, In the case of a line printer, this control problem may be viewed as being composed of a spatial control component along the tangential or "line printing" axis of the printer and of a timing component along its sagittal or "cross-line" axis.